Type 1 diabetes mellitus (T1DM) is an autoimmune systemic illness affecting approximately 1.25 million Americans.[1] Retinal consequences of diabetes have been attributed to nonenzymatic glycosylation of vasculature resulting in loss of vessel integrity. However, even before the development of retinopathy, patients present with visual field and contrast sensitivity changes.[2],[3],[4] Previous studies have proposed early neurodegenerative consequences of diabetes to mediate these effects. Consequences of diabetes such as elevated glucose and ganglion cell death alter important neuroprotective factors that lead to the acceleration of diabetic retinopathy. These mediators are important in anti-oxidative, anti-angiogenic, and anti-apoptotic functions. Loss of these key factors can exacerbate the development of diabetic neuropathy, resulting in neuronal death, oxidative damage, and increased vascularization.[5],[6]

Studies comparing the thickness of retinal layers in populations with and without diabetes have established the inner plexiform, macular, and outer temporal superior and inferior regions to be significantly thinner in those with T1DM.[7],[8] Although several biochemical changes have been proposed as possible etiologies for the death of ganglion cells, the initiating factor that catalyzes these biochemical changes in a patient with T1DM has yet to be noted.[9],[10]

This study examines the longitudinal changes in ganglion cell complex (GCC) thickness in the same cohort of patients diagnosed with T1DM over a 3-year period using the spectral-domain optical coherence tomography (SD-OCT) to quantify the progression of ganglion cell loss. A longitudinal examination of these changes has never been attempted before. Studies have examined longitudinal changes in GCC thickness in individuals without diabetes and have compared GCC thickness between individuals with T1DM and individuals without T1DM through a single time-frame analysis.[11],[12] However, this is the first examination where the main outcome focuses on the change in GCC thickness in the same, matched cohort of patients over a 3-year period. Changes in GCC thickness have been stratified in accordance to gender, duration of diabetes, body mass index (BMI), and age. We postulate that certain baseline characteristics will predispose patients to have significant losses in GCC thickness. Identification and appropriate management of these characteristics have the potential to minimize the loss of GCC.

Methods

Participants

This longitudinal study includes data from participants who have attended the “Friends for Life Conference” conducted at Orlando, Florida, during the 1st week of July, for the past 3 years: 2014, 2015, and 2016. At the conference, the telemedicine team from the Institute of Ophthalmology and Visual Science of New Jersey Medical School, Rutgers University, conducts retinal screenings. Previous setup of retinal screenings have been published.[13],[14] At the 2016 screening, in total, 218 attendees underwent a full retinal screening. Before the screening, individuals completed an intake sheet that documented age, gender, ethnicity, duration of diabetes, height, weight, hemoglobin A1c levels, blood sugar levels, etc. Informed consent was taken from patients and parents for patients who were under the age of 18 years. The study was approved by Rutgers University's Internal Review Board and met all HIPAA guidelines. Informed consent was obtained from all patients above the age of 18, and for patients younger than age 18 parental/guardian consent was obtained. The research followed the tenets of the Declaration of Helsinki.

Inclusion criteria for this study were 3 years of data using the SD-OCT from Optovue (Fremont, CA, USA) with good image quality for all 3 years as noted by a signal strength index (SSI) of ≥44. iWellness reports from previous years, 2015 and 2014, were presented only in an Excel format with SSI as the only consistent measure of image quality. Previous studies noted a positive correlation between a higher signal strength and better GCC measurements, with a cutoff of ≥44 being cited as optimal for GCC thickness measurements.[15] All measurements were taken without the use of mydriatic agents, and room lighting was kept at 80 candelas to maximize the natural dilation of pupils.

Apparatus

Spectral-domain optical coherence tomography specifics

The analysis discussed in this presentation focuses primarily on the findings from the SD-OCT. The SD-OCT utilizes near-infrared wavelength to capture a cross-sectional image of the retina while measuring the thickness of the retina, nerve fiber layer, GCC, and the cornea. All scans were processed with the use of the Optovue iVue SD-OCT iWellness software.[13] The iWellness software consists of (1) retinal thickness (central, superior, inferior, nasal, and temporal quadrants) and (2) GCC thickness (GCT; total, superior, and inferior).

Procedure

The retinal screenings consist of six stations. The first station measures blood pressure, pulse, visual acuity, and pupil size. The second station consisted of the autorefractor (Canon, RKF2 Tokyo, Japan) and the tonometer (Canon, TX 20 Tokyo, Japan). The third and fourth stations included the SD-OCT (Optovue, iVue, Fremont, CA, USA) and OCT angiography (Optovue, iVue, Fremont, California). The fifth station consisted of a nonmydriatic retinal camera (Canon, CR2 Plus-AF with EOS-60D Tokyo, Japan). The sixth station included an autoreader, EyeArt (EyeNuk, Irving, CA, USA) that detected diabetic retinopathy and provided severity scores according to the early treatment diabetic retinopathy study (ETDRS) standards and recommendations for further ophthalmologic referrals. The final station included two retinal readers who counseled patients on the imaging findings to better manage their control of diabetes.

Statistical analysis

IBM SPSS statistics (IBM, New York) and StatPlus (Microsoft, Washington) for Microsoft Excel were used. ANOVA repeated measures test was conducted to detect significant changes. The Grubbs' test was run to detect for outliers, none of which were found. The Shapiro–Wilk test was run to detect for normal data distribution which was met. Data are presented as mean ± standard deviation (SD) unless otherwise noted. P < 0.05 was considered statistically significant.

Results

In total, 39 individuals had consistent 3-year readings using the SD-OCT. Of the 39 individuals, two were removed for having SSI below 44 for at least one of the 3 years, resulting in a total inclusion of 37 individuals with 74 measurements, one from each eye.

The mean age of participants was 24.2 ± 4.5 years (SD). The mean duration of diabetes was 13.6 years ± 2.2 years (SD). The mean BMI of participants was 24.5 ± 3.2 (SD). There were 18 (48.6%) males and 19 (51.4%) females in our analysis. With regard to age breakdown, 21 (56.7%) participants were between the ages of 10 and 20, 11 (29.7%) participants were between the ages of 20 and 30 years, and 5 (13.7%) of participants were older than 30 years. With regard to BMI, 9 (24.3%) participants had BMIs <18.5, 13 (35.1%) participants had BMIs between 18.5 and 24.9, 8 (21.6%) participants had BMIs between 25.0 and 29.9, and 7 (18.9%) participants had BMIs >30. With regard to duration of diabetes, 15 (40.5%) participants had diabetes for <10 years, 16 (43.2%) participants had diabetes for between 10 and 20 years, and 6 (16.2%) participants had diabetes for >20 years.

There was a significant reduction in GCC thickness over the 3-year period from 2014 to 2016 for a subset of participants with certain baseline demographics. These include male gender, elevated BMI (25.0 and 29.9), patient aged 10–20 years, and duration of diabetes for 10–20 years [Figure 1]. The slopes of GCC loss were greater in all subgroups in comparison to previously published standards of patients without T1DM, highlighting an association between T1DM and GCC loss, especially in patients with certain baseline characteristics as specified above [Table 1].

Fundus images were evaluated for diabetic retinopathy by an expert grader through visual inspection [Table 2]. The following parameters were considered when assessing the fundus images: dot hemorrhages, flame hemorrhages, intraretinal microvascular abnormalities, and optic disc cup/disc ratio. Grading was completed based on the ETDRS classification.

Previous studies have established the correlation between decrease in GCC thickness and duration of diabetes.[7] However, this longitudinal study follows GCC changes over a 3-year period in the same subset of participants. Significant decreases in GCC thickness are found in males, those with BMIs 25.0–29.9, those who have had diabetes for 10–20 years, and those in the age group of 10–20.

To better understand the changes in our study, a comparison to normative data from studies following a longitudinal change in GCC thickness in individuals without T1DM is beneficial. A compilation of multiple studies with different mean age groups is referenced [Table 1].

In comparison with the studies referenced above, the slopes for all subgroups in our study with significant change are higher, indicating a greater decline in GCC thickness among individuals with T1DM compared to individuals without diabetes. However, it is important to note a decrease in the slope of GCC loss with increasing age. This observation cautions any direct comparison between our data and the studies mentioned above due to the dependence of GCC thinning rates on age. In our study, the average age of participants was 24 years with a range from 11 to 50; therefore, due to the large age variation within our study, incorporation of younger participants, and the age-dependent effects of GCC thickness loss, more conclusive statements can only be made after an age-matched study is performed with a population without T1DM. At present, no such age-matched reference data exist.

A negative trend was noted among all the groups with significant changes in GCC thickness. However, none of the participants exhibited any signs of proliferative or nonproliferative diabetic retinopathy (PDR) on a fundoscopic image. Only minor background diabetic retinopathy was present as shown in [Table 2]. This mirrors findings from other studies, indicating the development of visual field and contrast sensitivity defects even before the onset of diabetic retinopathy.[16],[17],[18] Neuronal death has been noted to occur before vascular changes. The results from this study corroborate these findings as death of neuronal cells evident through thinning of the GCC complex is noted even before the onset of any retinal changes on fundoscopic image, highlighting the importance of the SD-OCT in a complete retinal screening. Furthermore, the measurement of the GCC thickness provides an objective measurement that better quantifies the progression of disease compared to more subjective measurements such as visual field and contrast sensitivity testing.

Examination of the groups with significant changes demonstrates certain correlations regarding the progression of disease in reference to certain variables. In regard to gender, males in our study presented with more statistically significant GCC declines compared to females. A gender difference in the development of retinal changes has also been noted previously in many other studies.[19],[20],[21] Analysis of our results and comparison to previous literature does not provide conclusive results regarding whether males or females are more likely to develop retinal changes associated with diabetes, but demonstrates the existence of a gender disparity in relation to the development of diabetic retinopathy that needs further investigation.

There were also significant decreases in GCC thickness over a 3-year period in individuals with BMIs of 25.0–29.9, the range that corresponds with being overweight. This correlates with prior studies conducted on animal models which found a correlation between increased rate of ganglion cell death, thinner GCC thickness, and poor glycemic control.[22] Different participant characteristics can indicate poor glycemic control and studies have shown an inverse correlation between BMI and poor glycemic control, helping to explain the increased loss of GCC thickness in individuals with higher BMIs.[23]

The age of participants to have the most significant decline in our study was noted to be between the ages of 10 and 20 years. In other studies, higher rates of GCC loss were noted among younger individuals.[11] However, within this analysis, the highest rates of GCC thickness decline were noted in individuals aged 10–20 years, not the youngest individuals in our study, <10 years. Prior analysis regarding longitudinal changes in GCC thickness has not been extensively documented in younger individuals, limiting the conclusions from this observation.

Significant declines in GCC thickness was also noted in participants who have had diabetes for 10–20 years. This correlates with the time frame individuals first start to develop diabetic retinopathy. Other studies have noted the increase in probability of developing PDR from 0% in the first 5 years postdiagnosis to 27.9% 13–14 years postdiagnosis.[24] Once this peak is achieved at the 13– 14-year mark, there is a leveling off in the development of PDR. Although none of the participants in the study exhibited PDR, the increased likelihood of the development of PDR and loss of GCC thickness in individuals with a longer duration of diabetes can indicate an increased vulnerability of developing retinal changes during a certain period of disease progression.

There were certain limitations to this study. Patients' characteristics included in the study that were self-reported and thus subject to bias included height, weight, duration of diabetes, and age. Second, in certain subdivisions, the sample size was small and uneven throughout each criteria resulting in statistical bias. This limitation could not have been avoided due to the retrospective nature of the study. Third, GCC loss is a progressive pathological process that is better examined over a longer time frame. A longer time frame examination can better capture the pattern of GCC loss.

Another limitation that could not be controlled is the internal deviation within the OCT between each measurement that could have altered our interpretation of the data. The repeatability/reproducibility SD for the average GCC, superior average GCC, and inferior average GCC were 1.3, 1.4, and 1.3 μm, respectively.[25] The progression of GCC loss, although greater than the normative population without T1DM, is still a gradual process primarily due to the slow, lifelong progression of the pathology. A 3-year examination of these changes is a small time frame over the course of one's life; therefore, even minute variations in repeatability can significantly alter the data.

Conclusion

Significant GCC loss was noted in patients with T1DM. Subgroups that had significant losses include males, those who's BMIs were 25.0–29.9, and those who had diabetes for 10–20 years. Ganglion cell loss was noted before the presence of any diabetic retinopathy in patients with certain baseline characteristics, suggesting onset of neuronal loss before any vasculature changes. These factors can be used as predictors for progression of neuronal layer thinning. Neuronal changes can help explain the contrast sensitivity and visual field loss noted in patients with T1DM before the onset of any retinal vasculature changes.[28]